1. Field of the Invention
The invention relates to a charge transfer device to be used for a solid-state image sensor and a delay element, and also to a method of driving the same.
2. Description of the Related Art
A floating diffusion amplifying circuit (FDA) has been widely used as a circuit for detecting signal charges in a charge transfer device. For instance, such a floating diffusion amplifying circuit has been described in "Solid-State Image Sensor" published through Shokodo, 1986, pp. 73-77, and in "Solid-State Imaging with Charge-Coupled Devices" published through Kluwer Academic Publishers, 1995, pp. 76-79.
FIGS. 1A to 1C illustrate a conventional charge transfer device including FDA. The illustrated charge transfer device is of electron-transfer type. FIG. 1A is a top plan view of the charge transfer device, FIG. 1B is a cross-sectional view taken along the line 1B-1B in FIG. 1A, and FIG. 1C illustrates a channel potential at the time when the later mentioned n.sup.+ floating diffusion region 211 is reset. As illustrated in FIGS. 1A and 1B, the charge transfer device is comprised of a charge transfer section 218 including first pairs of transfer electrodes 261 and 262, and second pairs of transfer electrodes 271 and 272, first and second pairs of transfer electrodes being alternately arranged in a row, an output gate electrode 208 connected to a final stage of the charge transfer section 218, an n.sup.+ floating diffusion region 211, an amplifier 212 for amplifying a voltage in the n.sup.+ floating diffusion region 211, a reset gate electrode 209 for resetting the n.sup.+ floating diffusion region 211, a reset drain 210 constituted of an n.sup.+ diffusion region, and a p.sup.+ channel stopper 203 located around the active region where charges are transmitted.
The first and second pairs of transfer electrodes 261, 262, 271 and 272 are formed on an n-type well region 202 formed in a p-type silicon substrate 201 with a silicon dioxide film 205 being sandwiched between the transfer electrodes 261, 262, 271 and 272 and the n-type well region 202. A charge transfer channel is formed in the n-type well 202. An n.sup.- type region 204, namely a region in which n-type impurities are more lightly doped than the n-type well region 202, is formed below every two of the transfer electrodes 262 and 272 to thereby generate a difference in a channel potential.
A pulse voltage .PHI.H1 is applied to the first pairs of transfer electrodes 261 and 262, and a pulse voltage .PHI.H2 is applied to the second pairs of transfer electrodes 271 and 272. The pulse voltages .PHI.H1 and .PHI.H2 are alternately applied. That is, signal charges are transferred in two-phase drive.
The signal charges transferred to a final stage of the charge transfer channel is further transferred beyond a potential barrier formed below the output gate electrode 208 to the n.sup.+ floating diffusion region 211. As a result, a potential in the n.sup.+ floating diffusion region 211 varies. The potential in the n.sup.+ floating diffusion region 211 is amplified by the amplifier 212, and thereafter is output from the amplifier 212.
After the signal charges have been output in the above-mentioned manner, a potential in the n.sup.+ floating diffusion region 211 is reset. The n.sup.+ floating diffusion region 211 is reset by applying a high voltage to the reset gate electrode 209 to thereby cause a potential in the charge transfer channel located below the reset gate electrode 209 to be higher than a potential of the reset drain 210 with the result that a potential in the n.sup.+ floating diffusion region 211 is equal to a potential in the reset drain 210, as illustrated in FIG. 1C.
As mentioned above, in accordance with the conventional method of resetting the n.sup.+ floating diffusion region 211, it is absolutely necessary to cause a charge transfer channel located below the reset gate electrode 209 to have a higher potential than a potential in the reset drain 210 in order to equalize a potential in the n.sup.+ floating diffusion region 211 to a potential in the reset drain 210. If a charge transfer channel located below the reset gate electrode 209 had a lower potential than a potential in the reset drain 210, the n.sup.+ floating diffusion region 211 is improperly reset, resulting in an increase of noises.
Thus, it is quite important to cause a charge transfer channel located below the reset gate electrode 209 to have a higher potential than a potential in the reset drain 210 in order to properly reset the n.sup.+ floating diffusion region 211. Since an external circuit applies a predetermined potential to the reset drain 210, there is almost no fluctuation in a potential in the reset drain 210. On the other hand, a potential in a charge transfer channel located below the reset gate electrode 209 is dependent on impurities profile and/or a dimension of a reset gate. Hence, there tends to generate a dispersion in the potential in a charge transfer channel located below the reset gate electrode 209 due to errors in fabrication of the reset gate electrode 209.
It would be possible to apply a sufficient high voltage to the reset gate electrode 209 with a margin in order to keep a potential in a charge transfer channel located below the reset gate electrode 209 higher than a potential of the reset drain 210 for ensuring that the n.sup.+ floating diffusion region 211 is properly reset, even if there would be a dispersion in a potential in a charge transfer channel located below the reset gate electrode 209. However, if an amplitude of a drive voltage was reduced, it would be impossible to ensure an adequate margin, which would result in that the n.sup.+ floating diffusion region 211 is improperly reset, and hence, the charge transfer device would have a decreased yield.